This image shows the variations in the lunar gravity field as measured by NASA's Gravity Recovery and Interior Laboratory (GRAIL) during the primary mapping mission from March to May 2012.

What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?

Many exciting things were made known through robotics.

"We also know what the far-side of the moon looks like, how different Jupiter's Galilean satellites are and that there may be oceans beneath the surface of Europa."

For example, the moon's surface was solid and not like a place in a fairy tale where one would sink into nowhere. It is true that we did not find any aliens on the moon or Mars. However, we now know about the detailed structure of the atmosphere and ionosphere of Mars and Venus, the soil composition of the moon and Mars, the attributes of the many satellites of Jupiter and Saturn, and about the volcanoes on Jupiter's moon Io.

This image was taken by the Lunar Orbiter 2 spacecraft in 1966. This photo changed perceptions regarding the moon's surface by showing the moon to be a world with tremendous topography -- some of it Earth-like, much of it decidedly not.

Due to Io's volcanic activity, impact craters have disappeared on its surface. This image was taken by the Voyager 1 spacecraft.

We also know what the far-side of the moon looks like, how different Jupiter's Galilean satellites are and that there may be oceans beneath the surface of Europa. It sure was an exciting time or me: 1962 - 1999, of course it still is an exciting time for me.

Pictured here is a portion of the lunar farside.

This mosaic of a region in the northern hemisphere of Jupiter's moon, Europa, displays many of the features which are typical on the satellite's icy surface. Brown, linear (double) ridges extend prominently across the scene. They could be frozen remnants of cryovolcanic activity which occurred when water or partly molten water ice erupted on the Europan surface, freezing almost instantly in the extremely low temperatures found on this moon of Jupiter.

In your field of work, what are some examples of the great achievements and discoveries in planetary science and robotic exploration throughout the past 50 years?

Pictured here is a gravity map of the moon made by the Lunar Prospector spacecraft in 1998-99. Mascons are shown in orange-red. The finding of mascons on the moon in the late 1960s changed scientists ideas about the interior structure of the moon.

The most exciting planetary finding, for me, was the discovery of the Mascons on the moon and Mars with my colleague, Paul Muller; for it changed geologists and geophysicists' ideas about the interior structure of these bodies.

Apollo 17 image of Aitken Crater. Aitken Crater is about 135 km in diameter. Aitken crater sits on the northern rim of the South Pole-Aitken Basin, which is the largest (about 2,500 km in diameter) and oldest recognized impact basin on the moon.

Gravity highs were supposed to be in the mountains and highlands, and not in the lowlands or large basins with depressed topography -- a completely new phenomenon. And from this, many new ideas emerged to explain these results.